Poly(methyl methacrylate) has been known for many years as an useful material for a clear, weatherable thermoplastic. It can be prepared or formed into sheet or molded articles, it has excellent resistance to weathering, it has a high service temperature, preventing distortion on exposure to warm environments, it does not embrittle significantly on cooling, it has adequate toughness or impact resistance for many uses, surpassing glass in that respect, and it has excellent optical properties in its combination of light transmittance and avoidance of color. Such optical values are measured as % total white light transmittance, yellowness index, and haze.
Its major deficiency, as compared to materials like polycarbonate, has been lack of impact resistance. Many studies have been made to improve the impact resistance while maintaining the other desirable physical properties, including clarity. Most of these centered on the use of weatherable elastomeric materials, such as copolymers of alkyl acrylates, disperse in the poly(methyl methacrylate) matrix. The most commercially successful of these has been the use of a core/shell modifier structure, utilizing a butyl acrylate/styrene copolymer which has a refractive index matched to the poly(methyl methacrylate) matrix, and a grafted poly(methyl methacrylate) shell, the particle size being below 0.5 microns and the isolated particle dispersed in the poly(methyl methacrylate) matrix. A further improvement in the impact modifier has been the presence of a "hard core" of methyl methacrylate, chemically attached to the intermediate rubbery layer. The resulting polymer offers good optical properties, good resistance to crease-whitening on impact, and a five- to ten-fold increase in toughness over the unmodified matrix polymer.
The technology for such "core-shell" impact modifiers is described in Owens, U.S. Pat. No. 3,793,402, herein incorporated by reference. The technology for other core-shell modifiers is described in Owens, U.S. Pat. No. 3,808,130, also herein incorporated by reference. Many variants in the nature of such polymers may be utilized, including the order in which the various layers are created, the nature and extent of the grafting and/or crosslinking processes, the types of crosslinking monomers (defined in Owens as a monomer having two or more unsaturated sites of equal reactivity such as butylene glycol dimethacrylate, ethylene glycol diacrylate, and the like) and graftlinking monomers (a monomer having two or more sites of unequal reactivity, such as allyl methacrylate, diallyl maleate, and the like), the particle size, and the means of isolation. Also variations in the number and order of stages have been taught in several patent applications. It is anticipated that the present invention will be useful for all such variants.
The most convenient way to prepare such modifiers has been by sequential emulsion polymerization, as described in Owens, wherein the first stage or core of the multi-stage heterogeneous polymer is formed in emulsion, a second monomer or mixture of monomers is added under conditions which produce no new particles, so that the second monomer is polymerized on the surface or within the first polymer particle, the process being repeated until all the stages have been polymerized, the stages being attached to and intimately associated with the preceding stage. The additive is then usually isolated by spray-drying or coagulation and then blended with the poly(methyl methacrylate) matrix.
Because the matrix polymer is usually prepared by a bulk process in which few if any contaminants are present, whereas the emulsion process requires emulsifier, buffer salts, initiators which may be inorganic salts, and the like, the possibility exists that the isolation process, especially if it is spray-drying or coagulation under conditions where the emulsifier is not readily removed, will result in contaminants present in the impact modifier component which will affect the color or optical properties. Such contaminants may be insoluble precipitates which scatter light and cause haze, or they may be soluble components which contribute to color or which are sensitive to processing to form color.
A long-known and effective means for counteracting yellow color within a polymer sample has been to add very low levels of blue pigments or dyes, called toners. However, addition of toner will lower the amount of total white light transmittance, and, unless carefully controlled, will produce green or blue color.
Hung, U.S. Pat. No. 4,602,083, incorporated by reference, teaches that a variety of acrylic-based emulsions, including the impact modifier and the matrix polymer described herein, may be isolated from emulsion by the use of alkaline earth hypophosphites as coagulating agents, and that the resulting polymers have an improved combination of water resistance and optical properties, such as color, than similar materials isolated with conventional coagulants. Hung's process utilizes far larger quantities of the hypophosphite than are taught here, may also use other non-nucleophilic anions which are not reductants, and is not applicable to isolation of the impact modifier component by spray-drying. Hung does not suggest the utility in color reduction of the substantially lower amounts of reductant used in the present invention.
A means to lower the levels of both insoluble haze-producing and soluble color-producing contaminants has long been sought, and it is discovery of an method for improving the color without contributing to haze that is the present invention.